One type of a torque converter is described in U.S. Pat. No. 4,180,978. This torque converter includes a pump impeller having a plurality of blades, a turbine runner, and a stator. The pump impeller is driven by the internal engine and is adapted to absorb torque from the engine. The turbine runner is in fluid flow relation with the pump impeller. The stator redirects fluid from the turbine runner to the pump to produce a toroidal flow path of hydraulic fluid. The blades of the pump impeller have a negative tip angle displaced in the range of 20 degrees to 30 degrees from the toroidal flow path at the fluid outlet of the pump impeller. Further, the blades of the stator have a sharp nose displaced circumferentially and angularly from the entrance flow to the stator from the turbine runner to induce turbulence in the toroidal flow path at the stator inlet at stall speeds.
In the apparatus, when the speed ratio E is small, that is to say that the turbine runner does not rotate in spite of rotating the pump impeller, the torque capacity C becomes small. The speed ratio E is the rotational speed of the turbine runner divided by the rotational speed of the pump impeller. The torque capacity C is equal to Tp/Ne.sup.2, where Tp equals the pump impeller torque and Ne equals the rotational number. The torque capacity C is one of the factors deciding the peak torque of the engine. Therefore, when the engine is idling, the fuel expense of the engine becomes economical.
However, in the apparatus described above, the torque capacity C is not controlled when the speed ratio is in both the middle range and the high range. Therefore, the peak torque of the engine become small such that the fuel expense is not economical.
A need thus exists for a torque converter in which during a middle and high range of operation, the torque capacity is controlled to be generally constant and less than in other known torque converters.